iOS: Thread-unsafe Dictionary access in TileStoreController causes SIGSEGV crash

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## Environment

- **mapbox_maps_flutter**: 2.10.0 (also confirmed unpatched on `main` as of 2.21.0-SNAPSHOT)
- **Device**: iPhone 17,2 (iPhone 16 Pro Max)
- **iOS**: 26.4 (23E246)
- **Flutter**: 3.35.7
- **Distribution**: TestFlight

## Bug Description

`TileStoreController.loadTileRegion` crashes with `EXC_BAD_ACCESS (SIGSEGV)` due to unsynchronized concurrent access to the `tileRegionLoadProgressHandlers` Swift Dictionary from multiple MapboxCommon background threads.

We have **two independent crash reports** from the same device/build, both crashing at `TileStoreController.swift:33`, the `removeValue(forKey:)` call inside the completion closure.

## Root Cause

In [`TileStoreController.swift`](https://github.com/mapbox/mapbox-maps-flutter/blob/main/ios/mapbox_maps_flutter/Sources/mapbox_maps_flutter/Classes/Offline/TileStoreController.swift#L28-L34):

```swift
tileStore.loadTileRegion(forId: id, loadOptions: loadOptions) { [weak self] progress in
guard let self else { return }
// READ from Dictionary -- on MapboxCommon background thread
self.tileRegionLoadProgressHandlers[id]?.eventSink?(progress.toFLTTileRegionLoadProgress().toList())
} completion: { [weak self] result in
executeOnMainThread(completion)(result.map { $0.toFLTTileRegion() })
// WRITE to Dictionary -- also on MapboxCommon background thread (NOT main thread)
self?.tileRegionLoadProgressHandlers.removeValue(forKey: id) // <- line 33: crash site
}
```

Two problems:

1. **`removeValue(forKey:)` (line 33) executes on the MapboxCommon callback thread**, not on the main thread. `executeOnMainThread` only wraps `completion`, the `removeValue` call that follows runs on the original background thread.

2. **Progress callbacks (line 30) also read from the same Dictionary on background threads.** When multiple tile regions are downloading, a progress read on one thread and a completion write on another thread race on the same Dictionary.

Swift's `Dictionary` is a value type with copy-on-write semantics and is **not thread-safe** for concurrent read+write access. This results in `EXC_BAD_ACCESS`.

The same pattern exists for `tileRegionEstimateProgressHandlers` in `estimateTileRegion` (lines 54-59).

## Crash Signatures

**Crash 1** -- concurrent read during mutation:
```
Thread 29 Crashed:
0 Runner specialized __RawDictionaryStorage.find(_:)
1 Runner specialized Dictionary._Variant.removeValue(forKey:)
2 Runner specialized Dictionary.removeValue(forKey:) (TileStoreController.swift:33)
3 Runner closure #2 in TileStoreController.loadTileRegion(id:loadOptions:completion:)
4-6 Runner
7+ MapboxCommon (background worker thread)
```

Full crash logs available on request (two `.crash` files from the same device, ~30 minutes apart).

Start in ios/mapbox_maps_flutter/Sources/mapbox_maps_flutter/Classes/Offline/TileStoreController.swift, focusing on loadTileRegion and estimateTileRegion and their progress-handler dictionaries. Trace the progress and completion callbacks, then ensure all dictionary access is serialized consistently. Done means both handlers are protected from concurrent reads and writes during overlapping tile-region downloads.

Written by the indexing model from the issue text.